Thiamine, or vitamin B1, is an essential micronutrient required for several metabolic processes in living cells. Except for thiamine producers, all organisms rely on dietary intake and insufficient supply can lead to deficiency. Since the 1970s, thiamine deficiency outbreaks have been episodically reported across the Northern Hemisphere, particularly affecting high trophic-level taxa, such as seabirds and top predator fish. Salmonids are especially vulnerable to this deficiency which causes high mortality among offspring. Despite extensive research and several hypotheses proposed, a single underlying cause behind the onset of this deficiency remains unidentified.

This thesis aims to provide a comprehensive understanding of thiamine status of Atlantic salmon (Salmo salar) populations across systems with (Baltic Sea) and without (Lake Vänern, North Atlantic Ocean) documented thiamine deficiency events. It explores thiamine dynamics throughout the adult life cycle and potential influencing factors, including diet, fitness variables, and gut microbiota diversity. Results of these analyses were used in a life-history theoretical model for optimal thiamine allocation between tissues.

Lake Vänern population exhibited the highest thiamine status, followed by North Atlantic and Baltic Sea populations. These differences might reflect different thiamine availability and diet due to different feeding grounds. However, thiamine status did not correlate with salmon fatty acid profiles, as previously proposed in literature, indicating that other factors might modulate thiamine concentrations. Interestingly, thiamine generally decreased as salmon approached spawning in both Baltic and North Atlantic populations. This reduction, as shown by the data and modelling with and without starvation, was likely a natural consequence of fasting rather than to be related to thiamine deficiency within the system. Moreover, changes in the salmon fatty acid profiles throughout their lifecycle were consistent for both low-thiamine populations (Baltic Sea) and intermediate-thiamine populations (North Atlantic), suggesting that these changes might not be involved in thiamine deficiency development. The gut microbiota analysis revealed that their composition varied across the studied systems, but microbiota species diversity showed little to no relationship to thiamine concentrations. However, thiamine-synthesizing microbial taxa tended to be more prevalent in salmon populations with high and intermediate thiamine status, suggesting a potential role in positively modulating the host’s thiamine status.

This research provides novel insights into the thiamine dynamics of Atlantic salmon, highlighting the complexity of factors influencing the thiamine status.